Fix unintuitive behaviour of RegisterAt with custom registry (#181)

This commit fixes some counterintuitive behaviour when creating the middleware
with a custom registry.

Previously, using a custom registry with additional metrics requires avoiding
the RegisterAt function, which used only the prometheus.DefaultGatherer.

Given a *prometheus.Registry, instead of the more intuitive:

```
fp := fiberprometheus.NewWithRegistry(reg, "my-application", "http", "", nil)
fp.RegisterAt(app, "/metrics")
```

One would have to do this:

```
app.Get("/metrics", adaptor.HTTPHandler(promhttp.HandlerFor(reg, promhttp.HandlerOpts{})))
```

The change here detects when the `prometheus.Registerer` is also a
`prometheus.Gatherer`, and then uses the `prometheus.Gatherer` when calling
`RegisterAt`.

This change will _not_ fix work in the case of a custom implementation of
`prometheus.Registerer` when the that implementation is not _also_ a
`prometheus.Gatherer`, which is an unlikely situation for inexperienced users of
prometheus.

For the more common case of passing a `*prometheus.Registry`, this change will
provide a more intuitive experience.

middleware.go

@@ -34,6 +34,7 @@ import (
 
 // FiberPrometheus ...
 type FiberPrometheus struct {
+	gatherer        prometheus.Gatherer
 	requestsTotal   *prometheus.CounterVec
 	requestDuration *prometheus.HistogramVec
 	requestInFlight *prometheus.GaugeVec
@@ -107,7 +108,15 @@ func create(registry prometheus.Registerer, serviceName, namespace, subsystem st
 		ConstLabels: constLabels,
 	}, []string{"method"})
 
+	// If the registerer is also a gatherer, use it, falling back to the
+	// DefaultGatherer.
+	gatherer, ok := registry.(prometheus.Gatherer)
+	if !ok {
+		gatherer = prometheus.DefaultGatherer
+	}
+
 	return &FiberPrometheus{
+		gatherer:        gatherer,
 		requestsTotal:   counter,
 		requestDuration: histogram,
 		requestInFlight: gauge,
@@ -160,7 +169,7 @@ func NewWithRegistry(registry prometheus.Registerer, serviceName, namespace, sub
 func (ps *FiberPrometheus) RegisterAt(app fiber.Router, url string, handlers ...fiber.Handler) {
 	ps.defaultURL = url
 
-	h := append(handlers, adaptor.HTTPHandler(promhttp.Handler()))
+	h := append(handlers, adaptor.HTTPHandler(promhttp.HandlerFor(ps.gatherer, promhttp.HandlerOpts{})))
 	app.Get(ps.defaultURL, h...)
 }
 

middleware_test.go

@@ -22,6 +22,7 @@
 package fiberprometheus
 
 import (
+	"fmt"
 	"io"
 	"net/http/httptest"
 	"strings"
@@ -30,6 +31,7 @@ import (
 	"github.com/gofiber/fiber/v2"
 	"github.com/gofiber/fiber/v2/middleware/basicauth"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
@@ -332,3 +334,47 @@ func TestMiddlewareWithCustomRegistry(t *testing.T) {
 		t.Errorf("got %s; want %s", got, want)
 	}
 }
+
+func TestCustomRegistryRegisterAt(t *testing.T) {
+	app := fiber.New()
+	registry := prometheus.NewRegistry()
+	registry.Register(collectors.NewGoCollector())
+	registry.Register(collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}))
+	fpCustom := NewWithRegistry(registry, "custom-registry", "custom_name", "http", nil)
+	fpCustom.RegisterAt(app, "/metrics")
+
+	app.Use(fpCustom.Middleware)
+
+	app.Get("/", func(c *fiber.Ctx) error {
+		return c.SendString("Hello, world!")
+	})
+	req := httptest.NewRequest("GET", "/", nil)
+	res, err := app.Test(req, -1)
+	if err != nil {
+		t.Fatal(fmt.Errorf("GET / failed: %w", err))
+	}
+	defer res.Body.Close()
+	if res.StatusCode != 200 {
+		t.Fatal(fmt.Errorf("GET /: Status=%d", res.StatusCode))
+	}
+
+	req = httptest.NewRequest("GET", "/metrics", nil)
+	resMetr, err := app.Test(req, -1)
+	if err != nil {
+		t.Fatal(fmt.Errorf("GET /metrics failed: %W", err))
+	}
+	defer resMetr.Body.Close()
+	if res.StatusCode != 200 {
+		t.Fatal(fmt.Errorf("GET /metrics: Status=%d", resMetr.StatusCode))
+	}
+	body, err := io.ReadAll(resMetr.Body)
+	if err != nil {
+		t.Fatal(fmt.Errorf("GET /metrics: read body: %w", err))
+	}
+	got := string(body)
+
+	want := `custom_name_http_requests_total{method="GET",path="/",service="custom-registry",status_code="200"} 1`
+	if !strings.Contains(got, want) {
+		t.Errorf("got %s; want %s", got, want)
+	}
+}